The present invention relates to a tandem mass spectrometry method and device.
Mass spectrometry (MS) is an analysis technique for detecting ions originating from a sample and for analyzing these ions based on their ratio (m/z), wherein m represents the mass of an ion and z represents its electric charge. Mass spectrometry is used in numerous applications for analyzing, identifying, and characterizing the chemical structure of ionized molecules.
A mass spectrometer generally includes an ionization source for forming the ions from a sample to be analyzed, an analyzer that separates the ions based on their m/z ratio, and a detector. A mass spectrum is produced by recording the ion abundance based on their mass-to-charge (m/z) ratio. However, simple mass spectrometry does not always make it possible to differentiate ions that have identical m/z ratios, particularly in complex molecules.
Tandem mass spectrometry is an ion analysis method that consists of selecting an ion via an initial mass spectrometry step, fragmenting it, then performing one or more other mass spectrometry step(s) on the ion fragments thereby generated, wherein the mass analysis steps can be spatially or temporally separated. Tandem mass spectrometry can be performed by isolating an ion inside an ion trap, then by supplying it with a sufficient quantity of internal energy for it to fragment: this step is referred to as activation. Detection of the products of this fragmentation can provide data on the structure of the parent ion. Tandem mass spectrometry is the foundation for mass spectrometry applications in structural analysis and in particular for sequencing proteins and other biopolymers (such as sugars or nucleic acids).
Various activation methods for fragmenting ions exist. Each activation method involves various activation means that can lead to various activation products.
The most widely-used ion activation method is referred to as CID, for “Collision-Induced Dissociation.” Activation via CID consists of activating ions by inelastic collision between the ions and neutral target species, such as atoms or molecules of a rare gas (helium, nitrogen, argon, etc.). It consists of converting part of the ion's kinetic energy into internal energy. This method belongs to the class of vibrational activation methods, which are similar to slowly heating the ion. Despite its popularity, CID activation suffers from disadvantages. First, as a result of the collisions between ions and gas molecules, the trajectories of the ions can be modified. Hence, the CID step can lead to ion loss and decreased detector resolution. As a result of CID, competition occurs inside the ion trap between ion activation and ejection. Moreover, CID activation produces nonselective ion excitation: all of the ions present inside the ion trap can be excited by colliding with the gas. Finally, the efficacy of this method decreases as the mass-to-charge ratio of the ions increases. The mechanisms brought into play by CID are statistical and can cause the most fragile bonds to rupture. Therefore, CID does not make it possible to analyze certain ions with high m/z ratios or to obtain sequence data for certain molecules with fragile bonds.
A fragmentation technique using RF electromagnetic radiation is also known. US2005/009172A1 describes a tandem mass spectrometer for analyzing nonionized gas molecules that includes an ionization chamber, a VUV lamp for ionizing the gas molecules, an ion trap, an ion fragmentation unit inside the ion trap, and a time-of-flight mass analyzer for detecting the selected ions inside the ion trap. US2005/009172A1 states that the photon energy of the VUV lamp is sufficient for ionizing neutral molecules but insufficient for producing a fragmentation or dissociation beyond the ionization potential. According to this document, the ion fragmentation unit is composed of an electromagnetic radiation source, referred to as TICKLE, coupled to the ion trap.
Another method involving activation by laser is also known. EP1829082 describes the use, in tandem mass spectrometry, of a laser emitting in the visible range and near ultraviolet. The ions can absorb the energy of the laser beam photons. In principle, a selective activation can be generated based on the laser's emission wavelength. However, the available laser wavelengths are limited to the visible and to near ultraviolet and have limited photon energy at approximately 6.2 eV (or 200 nm).